CTQP Drilled Shaft Inspection — Questions and Answers
Question 1: During drilled shaft excavation using the wet method, what is the PRIMARY purpose of maintaining drilling slurry in the borehole?
- To provide hydrostatic pressure that prevents the borehole walls from caving in (Correct answer)
- To lubricate the drill tooling for faster penetration
- To carry drill cuttings to the surface for gradation testing
- To cool the drill bit and prevent overheating
Correct answer: To provide hydrostatic pressure that prevents the borehole walls from caving in
Drilling slurry (bentonite or polymer) exerts hydrostatic pressure against the borehole walls equal to or greater than the surrounding groundwater and soil pressure, preventing wall collapse in cohesionless or water-bearing soils. The other functions are secondary or incidental.
Question 2: When placing concrete in a drilled shaft using the tremie method, what is the minimum required embedment depth of the tremie pipe into the fresh concrete during placement?
- 1 foot
- 3 feet
- 5 feet (Correct answer)
- 10 feet
Correct answer: 5 feet
FDOT requires the tremie pipe to remain embedded at least 5 feet into the rising concrete mass. This ensures concrete is deposited from below the existing surface, preventing slurry from being trapped inside the shaft. Lifting the pipe above the concrete allows contaminated slurry to flow back into the placement.
Question 3: Cross-Hole Sonic Logging (CSL) is performed on completed drilled shafts to primarily:
- Measure the 28-day compressive strength of the shaft concrete
- Detect anomalies, voids, or defects within the concrete shaft (Correct answer)
- Determine the actual load-bearing capacity of the shaft
- Verify that the rebar cage is centered within the shaft
Correct answer: Detect anomalies, voids, or defects within the concrete shaft
CSL uses paired access tubes cast into the shaft. Ultrasonic pulses travel between transmitter and receiver tubes; delays or signal losses indicate zones of poor concrete, voids, or contamination. It is an integrity test, not a strength or capacity test.
Question 4: When inspecting a drilled shaft rebar cage before lowering it into the excavation, which item is MOST critical to verify?
- The color coding on the tie wire used to assemble the cage
- The manufacturer's mill certification markings on each bar
- The number, type, and spacing of concrete cover centralizers (Correct answer)
- The thread pattern on any mechanical bar splices
Correct answer: The number, type, and spacing of concrete cover centralizers
Centralizers maintain minimum concrete cover around the entire rebar cage as it is lowered and as concrete is placed. Insufficient or missing centralizers allow the cage to shift, reducing cover and potentially creating structural defects. Cover is a primary durability and structural requirement for drilled shafts.
Question 5: The inspector observes loose cuttings and disturbed soil at the bottom of a completed drilled shaft excavation. What should happen BEFORE the rebar cage and concrete are placed?
- Proceed with concrete placement — the weight of concrete will consolidate the loose material
- Place a layer of gravel at the bottom to stabilize the loose material
- The shaft bottom must be cleaned to remove all loose cuttings and debris (Correct answer)
- Add extra concrete yardage to the mix design to compensate
Correct answer: The shaft bottom must be cleaned to remove all loose cuttings and debris
Loose material, cuttings, and debris at the shaft bottom significantly reduce end-bearing capacity and can cause voids in the lower portion of the shaft. FDOT specifications require the shaft bottom to be clean before any reinforcement or concrete is placed.
Question 6: During drilled shaft construction, a polymer drilling slurry sample is taken and tested. Which property is routinely measured in the field to verify slurry acceptability prior to concrete placement?
- Compressive strength and air content
- Viscosity, density, and pH (Correct answer)
- Chloride content and water-cement ratio
- Bleed water percentage and initial set time
Correct answer: Viscosity, density, and pH
Field testing of polymer or bentonite slurry before concrete placement includes viscosity (Marsh cone), density (mud balance), and pH. These three parameters confirm the slurry can maintain borehole stability and will not contaminate or delay concrete hydration. FDOT specifications define acceptable ranges for all three.
CTQP Drilled Shaft Inspection
During drilled shaft excavation using the wet method, what is the PRIMARY purpose of maintaining drilling slurry in the borehole?